#include <iostream>

using namespace std;


class A {
public:
    int var1, var2;
    A(){
        var1 = 10;
        var2 = 20;
    }
    virtual void fun() = 0; // 纯虚函数
};

int main()
{
    A ex1; // error: cannot declare variable 'ex1' to be of abstract type 'A'
    return 0;
}